Energy autonomous sensors that harvest ambient energy from sources such as light, heat, vibration and radiation are promising solutions to provide long-term self-sustaining sensor operation without battery replacement. Among different ambient energy harvesting approaches, piezoelectric vibration energy harvesting attracts enormous research interest because of a relatively high energy density and a wide availability of environmental energy sources.
There have been different proposals and approaches to enhance the energy conversion efficiency of a piezoelectric energy harvester. To improve rectification efficiency, conventional MOS passive diodes have been proposed to be replaced with active diodes in significantly reducing forward voltage drop at the expense of additional comparator current. Switching converters have also been used to emulate optimum load resistance to achieve sub-optimum impedance matching for maximum power extraction. Nonlinear synchronized energy harvesting methods such as synchronized charge extraction (SCE) and synchronized switch harvesting on inductor (SSHI) are also explored to maximize output power.
However, the need of additional control and timing circuitries in the above proposals often require an external back-up energy source during start-up and this greatly reduces the self-startup capability of the energy harvester.
There is as such a desire for a piezoelectric energy harvester which provides an effective startup capability as well as rectification efficiency.